A polycarbonate resin plate is excellent in transparency, impact resistance and heat resistance, and is used for soundproof walls, carports, signboards, glazing materials, illumination devices and the like. However, a polycarbonate resin plate has a disadvantage of having a low surface hardness and is easy to be scratched, and thus the use thereof is limited.
In order to overcome the disadvantage, Patent Document 1 proposes a method of coating a surface of a polycarbonate resin plate with an ultraviolet-curable resin or the like, and a method of hard-coating a substrate obtained as a result of coextrusion of a polycarbonate resin and an acrylic resin.
However, a surface of a polycarbonate resin that is merely hard-coated does not have a required pencil hardness and is not usable for uses which require a certain level of surface hardness.
A method of coating a surface layer with an acrylic resin provides a surface hardness increased to some extent, and the resultant polycarbonate resin plate is usable for a wider range of uses including a front plate of an information display device and the like. However, the layer obtained by this method has a two-layer structure of different materials and may be warped when the environment is changed due to a difference in water absorption characteristic between the acrylic resin and the polycarbonate resin. Such a layer causes a flaw in a use in which an environmental change is involved.
As a laminate that is suppressed in warping, there is a laminate including acrylic resin layers provided on both of two surfaces of a polycarbonate resin layer. However, when a planar impact is applied to one of the surfaces of the laminate, the acrylic resin layer on the other surface of the laminate is easily cracked. This may cause a problem in a certain type of use.
As a laminate that is suppressed in warping, Patent Document 2 discloses a laminate including a metal methacrylate-styrene copolymer, which is a resin having a lower water absorption ratio than that of the acrylic resin, laminated on a polycarbonate resin. However, the conditions of 40° C./90% used in the environmental test are insufficient as high-temperature high-humidity conditions.
Patent Document 3 discloses a laminate including a highly hard denatured polycarbonate resin laminated on a polycarbonate resin, but does not refer to the water absorption characteristic or the shape stability against an environmental change.
Patent Document 4 discloses a laminate including a highly hard denatured polycarbonate resin different from that of Patent Document 3 laminated on a polycarbonate resin.